This invention relates in general to an electroforming process for using a reusable electroforming mandrel.
The fabrication of hollow articles by an electroforming process is well known. Generally, the articles are fabricated by electrodepositing a metal onto an elongated mandrel which is suspended in an electrolytic bath. The materials from which the mandrel and the electroformed article are fabricated are selected to exhibit a different coefficient of thermal expansion to permit removal of the belt from the mandrel upon cooling of the assembly. In one example of an electroforming arrangement, the mandrel comprises a core cylinder formed of aluminum which is overcoated with a thin layer of chromium and is supported and rotated in a bath of nickel sulfamate. In the process for forming large hollow articles having a large cross sectional area, it has been found that a diametric parting gap, i.e. a gap formed by the difference between the average inside electroformed article diameter and the average mandrel diameter at the parting temperature must be at least 8 mils (200 mm) and at least about 10-12 mils (0.250 mm-300 mm or 0.04-0.06 percent of the diameter of the mandrel) for reliable and rapid separation of the large article from the mandrel. For example, at a parting gap of about 6 mils (0.015 mm), high incidence of both belt and mandrel damage are encountered due to inability to effect separation of the belt from the mandrel. Various different techniques have been developed for forming and removing tubes from electroforming mandrels depending upon the cross sectional area of the electroformed tube.
A process for electroforming hollow nickel articles having a large cross sectional area onto a mandrel is described in U.S. Pat. No. 3,844,906 to R. E. Bailey et al. More specifically, the process involves establishing an electroforming zone comprising a nickel anode and a cathode comprising a support mandrel, the anode and cathode being separated by a nickel sulfamate solution maintained at a temperature of from about 140.degree. F. (60.degree. C.) to 150.degree. F. (65.5.degree. C.) and having a current density therein ranging from about 20 to 50 amps/ft.sup.2 (amps/dm.sup.2), imparting sufficient agitation to the solution to continuously expose the cathode to fresh solution, maintaining this solution within the zone at a stable equilibrium composition comprising:
______________________________________ Total Nickel 12.0 to 15.0 oz/gal (90-112.5 g/l) Halide as NiX.sub.2.6H.sub.2 O 0.11 to 0.23 mole/gal (0.44-0.92 mole/l) H.sub.3 BO.sub.3 4.5 to 6.0 oz/gal (33.7-45 g/l) ______________________________________
electrolytically removing metallic and organic impurities from the solution upon egress thereof from the electroforming zone, continuously charging to the solution about 1.0 to 2.0.times.10.sup.-4 moles of a stress reducing agent per mole of nickel electrolytically deposited from the solution, passing the solution through a filtering zone to remove any solid impurities therefrom, cooling the solution sufficiently to maintain the temperature within the electroforming zone upon recycle thereto at about 140.degree. F. (60.degree. C.) to 160.degree. F. (65.5.degree. C.) at the current density in the electroforming zone, and recycling the solution to the electroforming zone. The thin flexible endless nickel belt formed by this electrolytic process is recovered by cooling the nickel coated mandrel to effect the parting of the nickel belt from the mandrel due to different respective coefficients of thermal expansion.
For metal articles fabricated by electroforming on mandrels having a small cross-sectional area, the process described in U.S. Pat. No. 4,501,646 to W. G. Herbert overcomes difficulties in removing the electroformed article from the mandrel. For example, when the chromium coated aluminum mandrel described in U.S. Pat. No. 3,844,906 is fabricated into electroforming mandrels having very small diameters of less than about 1 inch, metal articles electroformed on these very small diameter mandrels are extremely difficult or even impossible to remove from the mandrel. Attempts to remove the small diameter electroformed article formed by the process described in U.S. Pat. No. 3,844,906 the can result in destruction or damage to the mandrel or the electroformed article, e.g. due to bending, scratching or denting. The entire disclosures of U.S. Pat. No. 3,844,906 and U.S. Pat. No. 4,501,646 are incorporated herein by reference.
Various materials may be utilized as temporary removable coatings from a mandrel. These coatings can be meltable to facilitate removal of an electroformed article thereon. For example, a coating of wax may be utilized as a meltable coating. However, a wax coating must be rendered conductive in order to function as an electroforming surface. The preparation of a conductive wax as well as the reclaiming of the conductive wax for subsequent use can be expensive and require additional processing steps. Moreover, wax is relatively soft and easily damaged during handling.
Japanese patent document No. J59-060702A, published Apr. 6, 1984, discloses a method of electroforming nickel stampers. The method comprises forming a thin reaction film having a low melting point on a base, embossing fine convex patterns on the film to make a master, forming a coating made of gold (alloy) on the master, forming a nickel layer by electrolytic deposition, separating a laminate comprising the gold and nickel from the master and eliminating the residual thin reaction films from the surface of the gold coat to obtain a nickel stamper. A reaction film is preferably made by reactive sputtering and preferably has a composition of TeCH. Residual film may be eliminated using ammonium persulfate. This process requires a reactive sputtering step, an embossing step, a gold deposition step and a special residual film elimination step using ammonium persulfate. The use of reactive sputtering equipment, exotic materials and multiple steps render the process complex and expensive. Moreover, since it appears that a reaction is necessary during sputtering to deposit the composition of TeCH, the deposited reaction film would appears to be unreusable.
Japanese patent document No. J5-8210 187 A relates to the formation of a parting film on the surface of a metal die, plating a layer on the parting film and peeling off the metal die to form a metal body having a configuration reversal to that of the metal die. The parting film is formed from an oxide film obtained by exposing the metal die to oxygen plasma. The oxygen plasma is applied to the surface of a substrate under oxygen pressure of 0.1-1 torr and a Ni film of about 0.2-0.3 mm in thickness is formed. The method is suitable for forming a pattern of highly integrated digital information, a deeply-grooved pattern, etc. in an electroforming process. Since the parting film is formed by the oxygen plasma, it is uniform and adhesive to the substrate. This process requires extremely high temperatures to form a parting layer and requires peeling off the metal die to form a metal body. Such an oxide film is difficult and expensive to make and does not appear to be readily reusable.
In U.S. Pat. No. 4,300,959, a method of electroforming is disclosed in which, for example, nickel and then copper is electroplated onto a wax mandrel coated with a sprayed coating of silver paint. A third layer of nickel is plated onto the copper layer and the wax mandrel is removed. The layered metallic object is heated to a point above the melting point of copper to yield a nickel electroformed object containing copper, nickel alloy. Since the wax substrate is not normally conductive, it is coated with a conductive silver coating which apparently comprises a heterogeneous mixture of silver particles, a binder , and a solvent for the binder so that the coating can be applied by spraying at room temerature to avoid melting the wax mandrel. The process disclosed in this U.S. patent requires the use of a mandrel that must be destroyed. Moreover, since the conductive coating on the wax would contaminate the wax, the coated mandrel constitutes a disposable, not a reusable material. In addition the use of an apparently heterogeneous conductive coating containing particulate material causes perturbations at the electroforming surface. In addition, a wax mandrel is highly vulnerable to surface damage prior to or during electroforming.
In order to reuse electroforming mandrels meeting precise tolerance requirements, the mandrels must be carefully handled to avoid scratching, gouging or otherwise damaging the smooth electroforming surface of the mandrel prior to, during and after each electroforming operation. Deep gouges, for example, may render the mandrel useless because electroformed articles may not be readily removed from the mandrel. Repair of damaged electroformed surfaces is normally impractical. Moreover, deformities in the surface of an electroforming mandrel can carry over to electroformed articles rendering electroformed articles useless for cosmetic reasons or for failing to meet the tolerance requirements for precision applications.